framing in roof over addition

I have gotten a lot of great advice from diychat and thought this would be a no brainer for seasoned framers/roofers.

The original house is only 16'6" wide, there was an addition on the backside with a flatroof. the addition is 9'1" I want to slope the roof over the addition.

my question is what size lumber should I use to fram the rafters? Would 2x6 suffice or should I go with 2x8"
here's what I got:
so on the front side of the house I have 8'3" to ridge with a 8/12 pitch

on the backside 9'1" was added so from the ridge to the outer rear wall is now 17'4"
I know the pitch will be shallower on the backside roughly about a 5/12.
The original trusses are the true 2x4" sized I know I need to use bigger dimmensional lumber with todays lumber.

I roughly calculated the length of the new rafter to be 19'4"
My question is will 2x6 be beefy enough using todays lumber (5 1/4") or should I pay the lil extra for overkill and go with 2x8" vbrep_register("46170")

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framing in roof over addition

Your lumber yard should have span tables for the species and grade of the lumber they carry and give you the dimension you need for that span. Or do a "Google" for lumber span tables and you'll be able to do it yourself.